Apparatus for manipulating a hot metal tundish on a gantry car

ABSTRACT

Invention includes a pair of saddle yoke assemblies mounted opposite each other in parallel spaced relation on cross supporting beams of a gantry car for receiving and supporting the trunnions of a hot metal tundish. The trunnions of the tundish are formed with a lower wedge shape portion and an upper rounded portion inwardly of the conventional cylindrical end of the trunnion. The wedge shape portions of the trunnions fit each into mating sockets or openings in a chuck in each of the saddle yoke assemblies. The chuck of each of the saddle yoke assemblies that receives the shaped portions of the trunnions is mounted for rotation in an upright trackway in the saddle assembly. The openings in the chucks are so dimensioned that when the wedge shape portion of each trunnion is received in the chuck, the rounded upper part of the trunnion forms a continuous circular surface with the circumference of the chuck so that when the chuck is rotated in the trackway while supporting the trunnions of the tundish, the rounded upper portion of the trunnion rides within the trackway and the tundish is prevented from falling from the yoke assembly when it is tilted to remove slag and steel residue from the tundish. The apparatus also includes means for raising and lowering the yoke saddle assemblies so as to lift the tundish high enough for the pouring spout or spouts depending therefrom to clear the top of a mold into which molten metal is to be cast. The apparatus is further designed to permit limited longitudinal movement of the saddle assembly along the two beams of the gantry car so as to properly position the pouring spout of the tundish relative to the interior of the mold.

United States Patent [1 1 Bode, Jr. et a1.

[ APPARATUS FOR MANIPULATING A HOT METAL TUNDISH ON A GANTRY CAR [75] Inventors: Charles H. Bode, .lr., Bethel Park;

Harry H. Britcher, Jr., Johnstown; Joseph E. McKelvey, Penn Hills Township, Allegheny City, all of Pa.

[73] Assignee: United States Steel Corporation,

Pittsburgh, Pa.

[22] Filed: Apr. 13, 11973 [21] App]. No: 350,844

I Related US. Application Data [63] Continuation-impart of Ser. No. 211,015, Dec. 22,

1971, abandoned.

[52] US. Cl 214/313, 105/163, 105/164, 141/232, 164/281, 214/314 [51] Int. Cl B22d 11/10, B65g 65/34 [58] Field of'Search..... 214/18 R, 313, 314, 18 PH; 222/166; 164/281; 266/39; 141/232 Primary Examiner-Robert G. Sheridan Attorney, Agent, or FirmDonald S. Ferito [5 7] ABSTRACT Invention includes a pair of saddle yoke assemblies mounted opposite each other in parallel spaced rela- Oct. 29, 1974 tion on cross supporting beams of a gantry car for receiving and supporting the trunnions of a hot metal tundish. The trunnions of the tundish are formed with a lower wedge shape portion and an upper rounded portion inwardly of the conventional cylindrical end of the trunnion. The wedge shape portions of the trunnions fit each into mating sockets or openings in a chuck in each of the saddle yoke assemblies. The chuck of each of the saddle yoke assemblies that receives the shaped portions of the trunnions is mounted for rotation in an upright trackway in the saddle assembly. The openings in the chucks are so dimensioned that when the wedge shape portion of each trunnion is received in the chuck, the rounded upper part of the trunnion forms a continuous circular surface with the circumference of the chuck so that when the chuck is rotated in the trackway while supporting the trunnions of the tundish, the rounded upper portion of the trunnion rides within the trackway and the tundish is prevented from falling from the yoke assembly when it is tilted to remove slag and steel residue from the tundish. The apparatus also includes means for raising and lowering the yoke: saddle assemblies so as to lift thetundish high enough for the pouring spout or spouts depending therefrom to clear the top of a mold into which molten metal is to be cast. The apparatus is further designed to permit limited longitudinal movement of the saddle assembly along the two beams of thegantry car so as to properly position the pouring spout of the tundish relative to the interior of the mold.

10 Claims, 8 Drawing Figures PATENTEMU 29 !9?4 SHEU 5 OF 7 FIG 5..

APPARATUS FOR MANIPULATING A HOT METAL TUNDISII ON A GANTRY QAR This application is a continuation-in-part of our copending application Ser. No. 21 1,015 filed Dec. 22, l97l, now abandoned.

The present invention relates to material handling equipment and more particularly, to apparatus for manipulating a tundish on a tundish supporting car, such as a gantry car, for properly positioning the tundish to discharge molten metal into a continuous casting mold. In continuous casting it is important that the tundish be held in such a position that the hot metal stream being discharged through the nozzle of the tundish is directed centrally into the receiving mold.

It is, accordingly, the primary object of this invention to provide apparatus for manipulating a tundish on a tundish car including means to raise and lower, adjust horizontally and tilt the tundish.

It is a more specific object of our invention to provide a tundish manipulating apparatus to be used on a wheeled gantry car. The apparatus includes a pair of vertically and horizontally adjustable tundish trunnion supporting means, one each on two cross supporting beams of the gantry car. Each of the trunnion supporting means includes a rotatable chuck or jaw for receiving and supporting the trunnions and tilting the tundish by rotating the trunnions.

These and other objects will become more apparent after referring to the following specification and drawing in which:

FIG. 1 is a plan view of a gantry car equipped with the apparatus of the invention;

FIG. 2 is an enlarged elevational view of FIG. 1;

FIG. 3 is a plan view of a saddle assembly portion of the invention;

FIG. 4 is an enlarged partial elevational view partly in section of FIG. 3;

FIG. 5 is a vertical sectional view taken substantially along the line V-V of FIG. 2;

FIG. 6 is a cross-sectional view of a tundish trunnion shaped in accordance with the invention;

FIG. 7 is a plan view showing a modified embodiment of the invention; and

FIG. 8 is an end view of FIG. 7.

Referring more particularly to the drawing, reference numeral 2 designates generally a wheeled gantry car'for transporting a hot metal tundish 4 along a pair of track rails 6 to and from a position over the mold M of a continuous caster. The gantry car 2 is conventional in structure and includes a substantially rectangular frame made up of spaced parallel longitudinal beams and spaced parallel cross supporting beams. The car 2 is provided with suitable self-propulsion means 8.

The apparatus of the invention, indicated generally by reference numeral 10, includes a pair of saddle assemblies 12 mounted opposite each other in alignment one on each of two adjacent cross supporting beams 14 and 16 of the gantry car frame for receiving and supporting the trunnions 18 of the hot metal tundish 4.

Each of the trunnions 18 is formed with an outer conventional cylindrical portion 22 which can be engaged by the J hooks (not shown) of a lift crane for placement on and removal from the gantry car 2. The portion of each trunnion adjacent the cylindrical portion 22 is formed with a depending wedge shape bottom 24 and a rounded top 26 for a purpose which will become apparent. The tundish is also provided with an overflow spout 28 projecting laterally from one side. The bottom of the tundish is provided with one or more discharge ports 30 which may have immersion pouring spouts (not shown) depending therefrom. Flow from the dis charge ports may be controlled by slide valves (not shown). Further, at the end of a cast it is desirable to be able to quickly tilt and dump the slag and steel residue from the tundish to avoid dlraining this material through the nozzle and gates in the bottom of the tundish. As will become apparent hereinafter, the apparatus of the invention provides novel means for quickly manipulating a tundish on a gantry car to achieve optimum continuous casting conditions and also effect quick and positive tilting of the tundish when required.

Both of the saddle assemblies 12 of the invention are similar so that only one will be described in detail.

Saddle assembly 12 includes a base plate 32 mounted on the cross supporting beam 14 with roller bearings 34 interposed therebetween to facilitate adjustable movement of the base plate longitudinally along the beam by means of a screw jack 36. Screw jack 36, which is mounted on the cross beam 14, is connected with the base plate 32 by means of an upstanding flange 38 on the end of the base plate. The roller bearings 34 are disposed for movement on platforms 40.

An upright U-shape frame housing 42 rests on the base plate 32 supported by and connected with the piston rods 44 of fluid pressure cylinders 46, one of which is disposed in upright position within each long leg 48 of the U-shape frame housing 42, as best shown in FIG. 4. The piston rods 44 are attached to the housing 42 by means of connecting pins 50. The head ends of cylinders 46 are fixedly mounted on the base plate 32.

Before proceeding with a detailed description of the apparatus of the invention, it may be well to interpose here a few remarks regarding continuous casting of metal generally. In continuous casting it is essential that the tundish be so positioned over the mold that the discharge port will direct the stream of hot metal issuing from the port into the mold in a vertical stream centrally of the interior of the mold. Otherwise, product of poor quality will be produced by the continuous caster. The tundish is moved horizontally to attain an optimum pouring position. Further, when the tundish is provided with pouring spouts, it is necessary to raise the tundish during its movement to a position over the mold so as to permit the pouring spout to clear the top of the mold.

Occasionally during a cast a malfunction will occur which necessitates stopping the discharge of molten metal into the mold. When this happens, the gantry car moves the tundish, with its discharge port closed, to a position over an emergency ladle or over an emergency trough leading to a ladle so that the metal may be removed from the tundish while it is still molten. Tilting of the tundish to quickly empty its contents by means of its overflow spout is frequently done.

An upstanding slotted guideway 52 is fixedly mounted on the base plate 32 adjacent opposite sides of the housing 42 for receiving and guiding flanges 54 which extend laterally from opposite sides of the housing 42. Roller bearings 56 may be provided in the guideways 52 spaced along the slots therein engageable by the edges of the flanges 54. Cylinders 46 are connected with suitable pressure fluid supply and exhaust means (not shown). Actuation of the cylinders 46 cause projection or retraction of piston rods 44 which in turn effect raising and lowering, respectively, as shown by broken lines in FIG. 4, of the housing 42 relative to the base plate 32.

The sides of the guideways 52 remote from the frame housing 42 are each provided with laterally extending flanges 58 which fit into upstanding guides 60 fixedly mounted on the frame of the gantry car. This arrangement insures retention of the frame housing 42 on the cross beam 14 when the base plate 32 is adjustably moved longitudinally along the cross beam.

An upstanding arcuate single-track trackway 62 is formed in the frame housing 42 in the center opening of or space between the long legs 48 thereof. The trackway 62 extends approximately 210 around the axis of the center opening of the frame housing 42. The upper part of the trackway is open and is attached to the long legs of the U-shape frame housing by gussets 64. The remaining portion of the trackway 62 is fixedly attached to the frame housing as by welding or similar means.

A substantially cylindrical chuck 66 is mounted for rotatable movement along the trackway 62 with a portion thereof extending beyond the frame housing 42. The chuck 66 is formed with a wedge shape opening 68 extending radially inwardly from its circumference for receiving the wedge shape bottom portion 24 of trunnion 18. The opening 68 is so dimensioned that when the portion 24 of trunnion 18 is inserted therein, the rounded top portion 26 of the trunnion rides along the trackway 62 when the chuck is rotated along the trackway to tilt the tundish 4 and thus the trunnion is retained in the chuck.

A substantially semi-circular plate 70 is bolted or otherwise rigidly affixed to the end of chuck 66 remote from its projecting end. The outer edge of plate 70 extends beyond the circumference of the chuck and slidably engages the side of the trackway 62. A rib 72 extends circumferentially around the body of the chuck 66 intermediate its ends and slidably engages the side of the trackway 62 opposite the side engaged by the plate 70. Thus, the plate 70 and rib 72 cooperate to limit transverse movement of the chuck 66 relative to the trackway.

A lever 74 is attached by one end to and extends tangentially from the projecting end of the chuck 66 and is pivotally attached by its other end to the piston rod 76 of a fluid pressure cylinder 78. The head end of the cylinder 78 is pivotally mounted on the frame of the gantry car 2. Cylinder 78 is connected with suitable supply and exhaust lines (not shown).

In operation, the tundish 4 carrying molten metal is placed on the gantry car by the .I hooks of a lift crane (not shown) with the wedge portions 24 of the trunnions 18 fitted into the wedge shape openings 68 of the chucks 66 of the opposed saddle assemblies 12. The J hooks of the lift crane are then disengaged and the cylinders 46 are actuated to project the piston rods 44 and lift the frame housings 42 upwardly, as shown by broken lines in FIG. 4. The gantry car is then propelled to position the tundish4 with a discharge port 30 thereof positioned over a mold M of a continuous caster assembly. If necessary, the screw jacks 36 are manually operated to move the saddle assemblies along the cross beams 14 and 16 to more accurately position the discharge port 30 centrally of the mold opening. Then the cylinders 46 are actuated to retract piston rods 44 and lower the frame housings 42 and the tundish toward the mold opening. The equipment is then in proper position to begin casting.

After casting has been completed, the cylinders 46 are actuated to project piston rods 44 and the gantry car is propelled to another site over a ladle. There cylinders 78 are actuated to rotate chuck 66 and tilt tundish 4 to dump the slag and metal residue therefrom. It will be understoodthat in event of an interruption of casting for any reason, the gantry car can move the tundish to a position over an emergency ladle or trough and the tundish can be tilted in similar manner to dump its contents while still molten.

FIGS. 7 and 8 show an arrangement wherein the apparatus of the invention is incorporated in another type of wheeled gantry car, designated generally by reference numeral 80. The apparatus of the invention shown in FIGS. 7 and 8 is designated with a similar reference numeral, but primed, to that used to designate the apparatus of the invention in the other Figures of the drawing. The gantry car 80 is conventional in structure and includes a substantially rectangular frame made up of spaced parallel upright cradle carrier frames 82 which extend transversely of the direction of travel of the gantry car at each end of the car. Parallel spaced beams 84 extend between and connect the ends of the cradle carriers 82. The car 80 is provided with selfpropulsion means (not shown) for movement along a pair of spaced parallel rails to and from a position over the mold M.

Each of the cradle carriers supports a cradle which includes an enlongated beam 86 extending transversely of the car ends. The cradles are each mounted for adjustable vertical movement in its respective carrier. The beams 86 are each formed with elongated horizontal center portions 88 and integral opposed ends 90 which extend upwardly and angularly away from the horizontal center portion, as best shown in FIG. 8. Each end 90 of each of the cross beams 86 is supported on a screw jack 92. The screw jacks 92 are connected together by power shafts 94 for operation by a motor 96 through a pair of gear boxes 98, one of which is connected with each of the screw jacks on the right ends of the cradle cross beams 86, as viewed in FIG. 7. It will be obvious that, if desired, each of the screw jacks could be independently powered.

The ends 90 of the cradles are provided with abutment portions 99 each having a cup 100 projecting from the bottom thereof for receiving the screw of the jack disposed therebelow. The abutments 99 on the left ends 90 of the cradle cross beams 86, as viewed in FIGS. 7 and 8, are provided with ribs 102 which project from three sides thereof, as best shown in FIG. 7. When the cradle cross beams 86 are raised or lowered by the jacks 92, the abutments 99 are guided by the ribs 102 traveling between guide rollers 104. The abutment portions on the right ends of the cradles, as viewed in FIGS. 7 and 8, are provided with projecting pads 106 which contact and roll long guide rollers 108. The guide rollers 104 and 108 are rotatably affixed to the cradle carriers 82 of the gantry car so that the cradle cross beams 86 can be raised and lowered relative to the gantry car by means of the screw jacks 92 to permit withdrawal and insertion of the pouring spout 110 of the tundish 4 from the mold M. The cradle cross beam 86 in FIG. 8 is shown in raised position by broken lines and in lowered position by solid lines.

It will be seen that the saddle assemblies 12' of the invention, which are mounted in alignment on the center portions 88 of the opposed cradle cross beams 86, can be raised and lowered by cooperative action of the screw jacks raising and lowering the cradle cross beams and action of the cylinders 46. If desired, actuation of the cylinders 46 may be omitted and only the screw jacks 92 may be used to raise and lower the saddle assemblies which in turn raise and lower the tundish as explained hereinbefore. Conversely, the cylinders 46 may be used solely for this purpose as shown in FIGS. 1 through 6.

Each of the saddle assemblies 12' is mounted for horizontal adjustment along its respective supporting beam by means of bolts 112 which pass through the base of the saddle assembly and through elongated slots (not shown) in the supporting beam. In operation, after the saddle assemblies 12' are properly positioned along the supporting beams, nuts (not shown) are tightened on the bolts 112 to rigidly affix the saddle assemblies to the supporting beams.

While we have shown but two embodiments of our invention, other adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. In a wheeled gantry car for supporting a tundish having trunnions projecting from opposite sides thereof mounted for limited horizontal and vertical movement on said car, said car having a frame with at least two spaced horizontal cross supporting beams, said cross beams extending transversely of the direction of travel of said car, the improvement therewith of apparatus for manipulating said tundish on said car, said apparatus comprising a pair of rotatable saddles mounted in upright position one on each of said cross beams substantially in alignment with each other, means connected with said cross beams for raising and lowering said saddles, said trunnions and said saddles having mating bearing surfaces whereby said trunnions rotate with said saddles to tilt said tundish when said saddles are rotated, means connected with said saddles for rotating the same, retaining means cooperable with said saddles for retaining said trunnions therein when said tundish is tilted, and means connected with each saddle for adjusting the same horizontally relative to its respective cross beam.

2. Apparatus as defined by claim l in which said two spaced horizontal cross supporting beams are mounted for vertical slidable movement in the frame of said car, and said means connected with said cross beams for raising and lowering said saddles includes four screw jacks disposed one under and connected with each end of each of said cross beams for raising and lowering the same.

3. Apparatus as defined by claim 2 including power means connected with said screw jacks for actuating the same.

4. In a wheeled gantry car for supporting a tundish having trunnions projecting from opposite sides thereof parallel with the path of travel of said car, said tundish being mounted for limited horizontal and vertical movement on said car, said car having a frame with at least two spaced horizontal cross supporting beams, said cross beams extending transversely of the direction of travel of said car, the improvement therewith of apparatus for manipulating said tundish on said car, said apparatus comprising a pair of saddle assemblies mounted one on each of said cross beams substantially in alignment with each other, each of said saddle assemblies including a base plate mounted on the cross beam, roller bearing means interposed between said base plate and its respective cross beam, means for adjusting said base plate longitudinally along its respective cross beam, a substantially U-shape frame disposed upright on said base plate, means mounted on said base plate and connected with said U-shape frame for moving the latter vertically relative to said base plate, an upright substantially circular trackway disposed in said U-shape frame, a substantially cylindrical saddle chuck mounted for rotation on said trackway with one end thereof projecting outwardly of its respective saddle assembly, said chuck having a cut-out portion for receiving a portion of a trunnion of said tundish, said portion of said trunnion and said cut-out portion having mating bearing surfaces, means connected with the projecting portion of said chuck for rotating the same along said trackway, and retaining means cooperating with said trackway to retain said chuck in said saddle assembly.

5. Apparatus as defined by claim 4 in which said cutout portions in said chucks are substantially in the shape of a keystone extending radially inwardly of said chuck circumference, and each of said portions of said trunnions is substantially keystone in shape with a rounded top, said portions of said trunnions each being dimensioned to fit into said cut-out portions of said chucks with said rounded tops adapted to move along said trackways when said chucks are rotated whereby said trunnions are retained in said chucks and said tundish is tilted when said chucks are rotated.

6. Apparatus as defined by claim 4 characterized by a lever projecting substantially tangentially from the projecting portions of each of said chucks, and said means for rotating said chucks including a pair of fluid pressure cylinders each having a piston rod projecting therefrom, the head end of each of said cylinders being pivotally mounted on said frame of said gantry car with the piston rod thereof pivotally connected with one of said levers whereby actuation of said cylinder projects and retracts said piston rod and rotates the chuck connected therewith.

7. Apparatus as defined by claim 4 in which said retaining means includes an arcuate plate affixed to one side of said chuck and slidably engaging one side of said trackway and a circumferential rib formed around said chuck intermediate the ends thereof slidably engaging the opposite side of said trackway to thereby limit transverse movement of said chuck relative to said trackway.

8. Apparatus as defined by claim 4 in which said means for vertically moving each of said U-shape frames including a pair of upright pressure fluid cylinders mounted on said base plate one in each of the upright legs of each U-shape frame, each of said cylinders having a piston rod projecting from the top thereof fixedly connected with its respective U-shape frame whereby simultaneous projection of said piston rods lifts said U-shape frames from said base plate.

9. Apparatus as defined by claim 4 in which each of said U-shape frames is formed with laterally extending flanges on opposite sides thereof, an upright slotted guide member is mounted on said base plate adjacent guide members, said rollers being adapted to engage Said flanges Said flanges fitting Slidably in the Slots of said flanges and facilitate vertical movement thereof in said guide members.

10. Apparatus as defined by claim 9 in which rollers gu'de membersare spaced along the interior of the slot of each of said 

1. In a wheeled gantry car for supporting a tundish having trunnions projecting from opposite sides thereof mounted for limited horizontal and vertical movement on said car, said car having a frame with at least two spaced horizontal cross supporting beams, said cross beams extending transversely of the direction of travel of said car, the improvement therewith of apparatus for manipulating said tundish on said car, said apparatus comprising a pair of rotatable saddles mounted in upright position one on each of said cross beams substantially in alignment with each other, means connected with said cross beams for raising and lowering said saddles, said trunnions and said saddles having mating bearing surfaces whereby said trunnions rotate with said saddles to tilt said tundish when said saddles are rotated, means connected with said saddles for rotating the same, retaining means cooperable with said saddles for retaining said trunnions therein when said tundish is tilted, and means connected with each saddle for adjusting the same horizontally relative to its respective cross beam.
 2. Apparatus as defined by claim 1 in which said two spaced horizontal cross supporting beams are mounted for vertical slidable movement in the frame of said car, and said means connected with said cross beams for raising and lowering said saddles includes four screw jacks disposed one under and connected with each end of each of said cross beams for raising and lowering the same.
 3. Apparatus as defined by claim 2 including power means connected with said screw jacks for actuating the same.
 4. In a wheeled gantry car for supporting a tundish having trunnions projecting from opposite sides thereof parallel with the path of travel of said car, said tundish being mounted for limited horizontal and vertical movement on said car, said car having a frame with at least two spaced horizontal cross supporting beams, said cross beams extending transversely of the direction of travel of said car, the improvement therewith of apparatus for manipulating said tundish on said car, said apparatus comprising a pair of saddle assemblies mounted one on each of said cross beams substantially in alignment with each other, each of said saddle assemblies including a base plate mounted on the cross beam, roller bearing means interposed between said base plate and its respective cross beam, means for adjusting said base plate longitudinally along its respective cross beam, a substantially U-shape frame disposed upright on said base plate, means mounted on said base plate and connected with said U-shape frame for moving the latter vertically rElative to said base plate, an upright substantially circular trackway disposed in said U-shape frame, a substantially cylindrical saddle chuck mounted for rotation on said trackway with one end thereof projecting outwardly of its respective saddle assembly, said chuck having a cut-out portion for receiving a portion of a trunnion of said tundish, said portion of said trunnion and said cut-out portion having mating bearing surfaces, means connected with the projecting portion of said chuck for rotating the same along said trackway, and retaining means cooperating with said trackway to retain said chuck in said saddle assembly.
 5. Apparatus as defined by claim 4 in which said cut-out portions in said chucks are substantially in the shape of a keystone extending radially inwardly of said chuck circumference, and each of said portions of said trunnions is substantially keystone in shape with a rounded top, said portions of said trunnions each being dimensioned to fit into said cut-out portions of said chucks with said rounded tops adapted to move along said trackways when said chucks are rotated whereby said trunnions are retained in said chucks and said tundish is tilted when said chucks are rotated.
 6. Apparatus as defined by claim 4 characterized by a lever projecting substantially tangentially from the projecting portions of each of said chucks, and said means for rotating said chucks including a pair of fluid pressure cylinders each having a piston rod projecting therefrom, the head end of each of said cylinders being pivotally mounted on said frame of said gantry car with the piston rod thereof pivotally connected with one of said levers whereby actuation of said cylinder projects and retracts said piston rod and rotates the chuck connected therewith.
 7. Apparatus as defined by claim 4 in which said retaining means includes an arcuate plate affixed to one side of said chuck and slidably engaging one side of said trackway and a circumferential rib formed around said chuck intermediate the ends thereof slidably engaging the opposite side of said trackway to thereby limit transverse movement of said chuck relative to said trackway.
 8. Apparatus as defined by claim 4 in which said means for vertically moving each of said U-shape frames including a pair of upright pressure fluid cylinders mounted on said base plate one in each of the upright legs of each U-shape frame, each of said cylinders having a piston rod projecting from the top thereof fixedly connected with its respective U-shape frame whereby simultaneous projection of said piston rods lifts said U-shape frames from said base plate.
 9. Apparatus as defined by claim 4 in which each of said U-shape frames is formed with laterally extending flanges on opposite sides thereof, an upright slotted guide member is mounted on said base plate adjacent said flanges, said flanges fitting slidably in the slots of said guide members.
 10. Apparatus as defined by claim 9 in which rollers are spaced along the interior of the slot of each of said guide members, said rollers being adapted to engage said flanges and facilitate vertical movement thereof in said guide members. 